1. Black Widow has a big and important role.
2. Give Hawkeye more to do and let Jeremy Renner show more of that cocky personality that he had in his Thor cameo.
3. Ultron being a menacing, badass villain.
4. Quicksilver and Scarlet Witch being portrayed well and having important roles in the movie.
5. Carol Danvers appearance to set her up for future films (my personal wish).